1. Field of the Invention
The present invention relates to a video signal transmitting system suitable for use in transmitting, for example, high vision signals (i.e., high definition signals) with a compressed frequency band.
2. Description of the Prior Art
For broadcasting a high vision (high definition) signal over one channel having a bandwidth of 27 MHz via satellite broadcasting wherein an FM modulation system is employed, the high definition television signal having a bandwidth ranging approximately from 20 to 25 MHz must be compressed to a bandwidth of about 9 MHz or less. For this purpose there has been developed the MUSE (Multiple Sub-Nyquist-Sampling Encoding) system which can compress the base band width of the high definition television signal to 8.1 MHz for broadcasting the high definition television signal over one channel of the satellite broadcasting system without substantially affecting the quality of the high definition image.
An encoder in accordance with the MUSE system performs frequency band compression of a video signal by converting an analog video signal to a digital video signal and then reducing the sampling rate of the sample pattern (known as sub-sampling). The video signal is represented by three dimensional axes, i.e., the horizontal axis, the vertical axis and the time direction axis, and sub-sampling can be performed along any of these three dimensional axes or along an arbitrary axis crossing these three dimensional axes. A 1/2 sub-sampling along any of these axes results in reducing the resolution of the video signal in the direction of that axis by a factor of 2.
The MUSE system makes use of the fact that the resolution of the human eye is lowered with respect to a moving image. Accordingly, the MUSE system detects whether each pixel is a still pixel or a moving pixel by use of a movement detector to adaptively change the sub-sampling system of the input signal which normally exhibits a sampling frequency of 48.6 MHz.
More specifically, a still pixel area, also known as a still image area, is subjected to the sequential processing of a field offset sub-sampling (hereinafter called "VOS") with a clock frequency of 24.3 MHz, followed by interpolation filtering at 12 MHz, followed by conversion of the sub-sampling frequency to 32.4 MHz and followed by a frame offset sub-sampling with a clock frequency of 16.2 MHz (hereinafter called "FOS"). When the number of horizontal scan lines per frame is odd, the FOS is equivalent to a line offset sub-sampling (hereinafter called "LOS"), so that the FOS may be regarded as frame/line offset sub-sampling (FOS/LOS). On the other hand, a moving pixel area, also known as a moving image area, is subjected to the sequential processing of band limitation by means of a low-pass filter having a frequency at 16 MHz, conversion of the sampling frequency to 32.4 MHz and line offset sub-sampling (LOS) with a clock frequency of 16.2 MHz. A reduced signal derived from a still image area and a reduced signal derived from a moving image area are generated for each of the respective pixels of the image, and these two reduced signals are weighted in accordance with the degree of change in pixel signals between frames and then mixed.
FIG. 1 shows a transmission bandwidth of a conventional MUSE system, wherein the abscissa represents the spatial frequency of a present image in the horizontal direction in units of the sampling frequency (MHz), while the ordinate represents the spatial frequency of the present image in the vertical direction in units of the number of c/ph (cycles/picture height) of horizontal scan lines in one frame as a unit. The sampling frequency in the horizontal direction in terms of MHz is the same as Msps (samples per second) which indicates the number of samples taken in the horizontal direction per second. The spatial frequency in the vertical direction may also be represented as a TV number, wherein 1 cycle per picture height is twice the TV number (i.e. 1 [c/ph]=2 [TV number]). Since one frame of a high definition signal includes 1,125 horizontal scan lines, an input MUSE signal has a sampling frequency of 48.5 MHz, and the field frequency of the high definition signal is set at 60 Hz. According to the Nyquist theorem, the upper limits of the transmission band in the vertical direction, the horizontal direction and the time direction are respectively 1125/2 cycles per picture height, 24.3 MHz and 30 Hz.
In FIG. 1, a substantially triangular area below line 1 indicates the transmission band of a still image area and a triangular area below line 2 indicates a transmission band of a moving image, wherein the resolution in an oblique direction in the still image area and the moving image area is reduced by a factor of 2 due to offset sub-sampling. Since one image is formed of 2 frames in the still image area 1 while one image is formed by interpolation in one field in the moving image area 2, maximum values of the frequencies (temporal frequencies) of the image movement in the time direction with which the image can be transmitted without distortion are 1/4 of the frame frequency (or 7.5 Hz) in the transmission band 1 of the still image area and 1/2 of the field frequency (or 30 Hz) in the transmission band 2 of the moving image area, respectively. However, in a horizontal frequency band below 4 MHz in the transmission band 1 of the still image area, the maximum value of the temporal frequency is 15 Hz since aliasing distortion due to the FOS is not produced.
In the conventional MUSE system as described above, an area in which the temporal frequency of movement is below 7.5 Hz is processed as a still image area, which leads to presenting good resolution in the horizontal, vertical and oblique directions. In contrast, an area in which the temporal frequency of movement exceeds 7.5 Hz is processed as a moving image area. However, since the transmission band 2 of the moving image area is limited, particularly in the oblique direction, the image of a slash having a relatively small pitch (for example, approximately 4.sqroot.2 or 6 in the horizontal scan line number), when vibrating at a frequency above 7.5 Hz, becomes dim.
Also, the resolution of the still image area and the moving image area in the oblique direction is sacrificed when performing band compression in the MUSE system as shown in FIG. 1, because of the fact that the resolution of the human eye is generally low in oblique directions. However, dim images in oblique directions may be observed, such as the above-mentioned slash.